Investigating plasticity of the olfactory system through the lens of specific anosmia.

The olfactory experiences vary widely among individuals. Despite having an otherwise normal sense of smell, some people exhibit specific anosmia, i.e., the inability to detect certain odorants. While genetic factors play a key role in this phenomenon, olfactory perception is also influenced by environment, and repeated exposure to odors (olfactory training) can enhance olfactory sensitivity. In this study, we examined whether olfactory training increases sensitivity towards odors participants were specifically anosmic to, and preliminary investigated whether this training-induced change is stable over time. Initially, we screened 335 participants with healthy sense of smell to identify individuals with specific anosmia towards androstenone, benzyl salicylate, bacdanol, or maltol. Subsequently, 77 participants with at least one specific anosmia underwent 2-months long olfactory training with these four odorants. We observed that following the training, participants became more sensitive towards odors they were specifically anosmic to, whereas sensitivity towards odors they were able to perceive at baseline did not change. However, the effects of olfactory training on specific anosmia were transient - 19 months after the training completion, 9 out of 10 followed-up participants became specifically anosmic towards androstenone again. Altogether, our findings demonstrate that the human olfactory system adjusts to novel odorous inputs, but these environmentally driven changes do not appear to be permanent in healthy participants.